The invention relates to an electrodeless low-pressure discharge lamp having a glass lamp vessel which is sealed in a gas-tight manner and which is filled with at least a metal vapour and a rare gas, said lamp having a first winding which is connected to a high-frequency electric power supply unit and which generates an electric discharge in the lamp vessel, one of the supply wires of the first winding being electrically connected to a supply wire of a second winding which extends at the area of the first winding and which has a free end, the potential gradient between the ends of the first winding being substantially equal to that of the second winding during operation Such a lamp is known from Netherlands Patent Application No. 8401307 laid open to public inspection.
This lamp, formed as a high-frequency operated fluorescent electrodeless low-pressure mercury vapour discharge lamp having a bulb-shaped lamp vessel, is used, inter alia as an alternative for an incandescent lamp for general illumination purposes.
In the above-mentioned lamp, the ends of the first winding (surrounding a rod-shaped ferrite core) are connected to a high-frequency oscillator power supply circuit, for example of a type as described in Netherlands Patent Application No. 8004175 laid open to public inspection. Such a circuit is comparatively simPle and during operation of the lamp one of the supply wires of the winding is permanently at a zero potential level. Due to the presence of a second winding, high-frequency electric interference currents occurring during lamp operation in the conductors of the power supply mains are reduced to an acceptable level. The number of turns of the two windings is preferably equal in order to obtain the same potential gradient (this is the potential decrease per unit length of the winding measured in the direction of its longitudinal axis). The presence of the second winding compensates for the electric interference currents, generated by the first winding, on the power supply mains.
The advantage of this known lamp is that it obviates the use of a transParent conducting layer on the inner wall of the lamp vessel, which layer is connected to one of the supply wires of the power supply mains for suppressing the said interference currents. Providing the layer and connecting the said mains conductor is complicated, timeconsuming and costly.
However, it has been found that the presence of the second winding adversely affects the ignition properties of the lamp. This can be ascribed to the fact that the lines of force of the generated electric field in the lamp vessel are contracted proximate the position where the windings are located As a result the ionization of the gas proceeds with greater difficulty. This is notably the case in lamps in which the power consumption is reduced (dimmed state).